[droidno9]

A couple of additions:

On non-disc road bikes, the tire size is limited by not only the width of the rims, but also by what the brake calipers can accommodate. For example, newer Shimano rim-brake calipers can accommodate tires up to 28mm in width, whereas older versions only up to 25mm in width. With disc bikes, the tire size is limited by what the wheel and frame can accommodate.

Gravel bike tires are all over the place, with companies making frames that can accommodate 700c, 29", and/or 27.5" wheels. And thus tire variations can be quite overwhelming.

[mauvehaus]

Frame clearance can get you too. Cantilever brakes will allow you to fit a lot of tire, but you'll likely rub the frame long before you rub the brakes.

[walrus01]

another "interesting" thing to consider is that some 700C gravel or cyclocross frames, designed for disc only, can take either 700C wheelsets or 650B. Because you no longer need to consider the vertical distance between the rim and the top of your fork, other than the general width between forks and rear stays for tire clearance.

650B is becoming increasingly popular for gravel.

There are a number of 650B tires that have intentionally higher than normal sidewall heights, so the total wheel+tire diameter is actually not that different from something like a 622x23 (aka 700C) skinny road bike tire on a traditional pavement only road bike.

https://off.road.cc/content/buying/13-of-the-best-650b-grave...

https://granfondo-cycling.com/650b-vs-700c/

If you have a disc only frame with wide tire clearance there exists the possibility of swapping between wheelsets that are either 650B or 700C depending on what type of riding you have planned in the near future.

[petre]

My all road bike was built with 650b wheels since I didn't want skinny tires. The issue is that with this wheel size the bottom bracket is about 15…30 mm lower compared to 700c so you'll have to be careful with crank length due to the increased risk of pedal strike. Most of these bikes have a bottom bracket drop of 70 mm or more, while a frame designed for 650b specifically will have 54…60 mm. I don't know why the industry doesn't build frames with 65 mm BB drop since it would be in between, but I'm inclined to believe they prioritize 700c with 40…50 mm tires. But then one wouldn't be able to use anorher wheelset with skinny 25 mm tires for road, so you'll probably need a road bike. While designing for single wheel size only, the industry also ignores the needs of shorter people and most women.

[sasawpg]

My cross bike will take up to a 38mm 700c tire but I can size up to 45mm 650b with no changes! Small miracle of dropping rim brakes.